The present invention relates to a method of making a semiconductor super-atom and an aggregate thereof. More particuarly, the present invention relates to a novel method of making a semiconductor super-atom and a method of making an aggregate of semiconductor super-atoms, that are useful for next-generation electronics such as computing devices for quantum computers, electronic elements highly variable by external fields, e.g. electric/magnetic field, and electrically conducting paths etc.
During late years, in the field of electronics, active studies are being conducted on next-generation electronics, such as computing devices for quantum computers, electronic elements highly variable by external fields of electric/magnetic field etc., ultra high-speed one dimensional electrically conducting paths, and new device materials are continuously being searched for, attempting to realize the next-generation electronics. Now , attracting attention as one type of such device materials, is an artificial giant quasi-atom, or super-atom, constituted by a nano-meter-scale 3-dimensional semiconductor hetero-junction structure, the concept of which had been proposed in 1986.
A semiconductor super-atom is defined as a structure where a singlecrystalline minute sphere (core) containing impurity atoms of donors or acceptors is embedded in a parent-phase singlecrystal without impurities and where the relationship between the parent-phase and the core is the reverse of a typical quantum dot structure. As an example thereof, the one utilizing GaAs as the parent-phase and AlGaAs as the core has already been proposed.
The internal mechanisms of a semiconductor super-atom have been made clear to a certain extent. That is, if the step at the edge hetero-interface of a conduction band (parent-phase) is sufficiently large, then donors (impurity atoms) are ionized and electrons shift to the conduction band of the parent-phase. Those electrons will be attracted to the positively charged donors, but are blocked by an interface barrier and they remain in proximity of the core. Although this phenomenon is the same as the well-known phenomenon in modulation-doped 2-dimensional electron system, if the diameter of the core is as minute as de Broglie wave length of electrons (approx. 10 nm), the electrons are quantized to have an atomic-like discrete level, so that this system may be viewed as a giant quasi-atom, or super-atom. Since, at this point, the core has overall electric charge of +Ze, Z would be the atomic number of the super atom.
There have been several methods proposed for making such a semiconductor super-atom, and the basic concept in those various methods is to form a semiconductor nano-structure as a core with a diameter in the order of 10 nm, and to add impurity atoms only to the semiconductor nano-structure, or core, with the number of the impurity atoms being controlled with a single-atom level accuracy. Also, by placing a number of the semiconductor super-atoms in an interval of 100 nm or less, an aggregate of semiconductor super-atoms can be formed.
The conventional methods that had been proposed, include;
1) a method employing single-atom manipulation technologies using field ion microscopy or scanning tunneling microscopy;
2) a method employing selective growth or selective etching of minute structures using a focused electron beam;
3) a method using ultra-fine particles; and
4) a method employing lithography on a multi-layered thin film.
However, the problems listed below have been pointed out to be present in these conventional methods, and the fabrication of a semiconductor super-atom is still difficult.
In the method of 1), it takes more than half a day only to form a single semiconductor super-atom, so that the throughput is extremely low, and also, it is difficult to process a plural number of atoms including impurities.
In the method of 2), the spatial resolution of the electron beam is low, and it has been unable to gain the spatial resolution sufficient for the formation of a semiconductor super-atom.
In the method of 3), impurity atoms cannot be added selectively only to the core portion.
In the method of 4), the spatial resolution of the lithography is low, and defects can be introduced to the interface during fine processing.
Therefore, although there proposed a concept of a new device material, namely semiconductor super-atom, which seems useful for the next-generation electronics, it has been difficult, and even almost impossible to form a semiconductor nano-structure as a core with a diameter in the order of 10 nm, and to add impurity atoms only to the core while controlling the number of the impurity atoms with a single-atom level accuracy.
Accordingly, an object of the invention is to provide a novel method for making a semiconductor super-atom and an aggregate thereof, which can solve the problems described above and realize the semiconductor super-atom and aggregate thereof.
The present invention provides, at first, a method of making a semiconductor super-atom, wherein impurity atoms are selectively introduced only to a core with the number of the impurity atoms being controlled with a single-atom level accuracy, by using droplet epitaxy for formation of a semiconductor nano-structure constituting the core and scanning tunneling microscopy for addition of the impurity atoms to the semiconductor nano-structure.
Secondly, the present invention provides a method of making a semiconductor super-atom wherein impurity atoms are selectively introduced only to a core with the number of the impurity atoms being controlled with a single-atom level accuracy, by using droplet epitaxy for formation of a semiconductor nano-structure constituting the core and a focused electron beam for addition of the impurity atoms to the semiconductor nano-structure.
Thirdly, the present invention provide a method of making a semiconductor super-atom wherein impurity atoms are selectively introduced only to a core with the number of the impurity atoms being controlled with a single-atom level accuracy, by using droplet epitaxy for formation of a semiconductor nano-structure constituting the core, and a technique to promote selective adsorption of the impurity atoms into a liquid droplet during the droplet epitaxy for addition of the impurity atoms to the semiconductor nano-structure.
Fourthly, the present invention provides a method of making a semiconductor super-atom wherein impurity atoms are selectively introduced only to a core with the number of the impurity atoms being controlled with a single-atom level accuracy, by using droplet epitaxy for formation of a semiconductor nano-structure constituting the core, and a technique to promote selective dissolution of the impurity atoms present at the substrate surface into a liquid droplet during the droplet epitaxy for addition of the impurity atoms to the semiconductor nano-structure.
Fifthly, the present invention provides a method of making an aggregate of semiconductor super-atoms by forming a plural number of semiconductor super-atoms on a substrate using any one of the above methods.